Agriculture Reference
In-Depth Information
canopy with a transmission coefficient (
K
)of
.
, Monteith (
) calculated
MJ m
−
day
−
could
double canopy dry matter production. In fruit trees with low LAI values the
effect is less but still appreciable. Wagenmakers (
that a doubling of above-canopy irradiance from
to
) estimated apple orchard
fruit production potential from canopy photosynthesis assuming a
-month
growing season with respiration costing
% of gross photosynthesis. With
a constant LAI of
, energy differences due to latitude alone would result
in fruit yields being about
.
◦
. Where cloudiness
is also taken into account the potential yield difference becomes
◦
than at
% higher at
%, i.e.
tha
−
per degree of latitude. If a higher LAI value (
about
.
instead of
.
)
◦
compared with
◦
, the relative potential yields become
is assumed for
tha
−
compared with about
tha
−
i.e. a ratio of
about
essentially due to irradiance intensity. Higher yields at lower latitudes are
further a result of higher temperature giving a longer growing season. These
conclusions are borne out by growth and yield data. Cripps (
.
to
) found that
shading to give
% of full sunlight in Western Australia, i.e. to levels still well
above those of northern Europe in energy terms, reduced total dry matter
production by
%. Folley (
) estimated that yields in southern France
were about
% higher than in southern England and, whereas absolute
maximum experimental yields recorded in Denmark and England are
and
tha
−
(Wagenmakers
), commercial orchards have attained yields of
tha
−
in New Zealand (McKenzie,
-
), some yields having reached
tha
−
. Experimental yields have reached
tha
−
in Tasmania ( Jotic,
tha
−
in Israel (Bravdo,
).
High irradiance levels are frequently, but not invariably, accompanied by
high temperatures. Night-time temperatures are generally lower in relation to
day-time temperatures and irradiation in arid areas. Day and, especially, night
temperatures are lower in relation to irradiation at higher altitudes. This may
influence the balance of respiratory losses to photosynthetic gain.
Many factors, e.g. leaf nitrogen status and water-stress effects on stomatal
behaviour influence the efficiency with which light energy is used in dry mat-
ter production. By and large they are optimized by management practices
developed to overcome obvious adverse effects on growth. Selection of culti-
vars, e.g. spur-types with thicker leaves and more palisade layers, giving higher
photosynthesis per unit area, may achieve increased photosynthesis without
a commensurate increase in within-canopy shade. This may also be achieved
by the use of plant growth regulators.
The net carbon budget for apple trees, defined as the difference between
the amount of CO
lost at night and gained during the day, follows the net
photosynthesis curve over the season, with a fairly consistent ratio of dark res-
piration to net photosynthesis in June, July, August and September of
) and at least
%
in western Europe. Dark respiration increases sharply in October, partly as a
-
